Direct current arc power supply

ABSTRACT

An arc power supply system having a three phase transformer and a full wave rectifier which includes silicon controlled rectifiers. A feedback control system includes first and second cascaded operational amplifiers. The first amplifier sums (1) a reference set point signal, (2) an adjustable voltage feedback signal and (3) an adjustable current feedback signal. The first amplifier provides low pass filtering to smooth the otherwise high ripple content of the two feedback signals. The second amplifier provides both integral and proportional control and drives a firing circuit. By the use of a single slope adjustment control, the current and voltage feedback signals may be adjusted to provide infinite and continuous control of the voltampere characteristic of the power supply between a constant current characteristic and a constant potential characteristic. An adjustable start circuit provides control of the average output voltage, and hence the initial arc energy, at the start of operation to more easily establish the arc.

OTHER PUBLICATIONS A. F. Manz, The One Knob Welder, Welding JournaI,Sept. 1968, Pp- 720-725.

Primary Examiner-J. V. Truhe Assistant ExaminerClifford C. ShawAttorney, Agent, or Firm-James C. Simmons; Barry Moyerman I UtutedStates Patent 1 I 1 3,896,287

Cook July 22, 1975 DIRECT CURRENT ARC POWER SUPPLY 57 ABSTRACT [75)Inventor: George E. Cook, Brentwood, Tenn.

[73] Assignee: Air Products and Chemicals, Inc., An arc power Supplysystem having a three phase I Allentown, Pa, transformer and a full waverectifier which includes silicon controlled rectifiers. A feedbackcontrol sys- [22] filed July 1973 tem includes first and second cascadedoperational [2-1;] A l N 378,265 amplifiers. The first amplifier sums(l) a reference set point signal, (2) an adjustable voltage feedbacksignal and (3) an adjustable current feedback signal. The 219/131 219/]first amplifier provides low pass filtering to smooth the 131 Rotherwise high ripple content of the two feedback sigre 0 care 219/135321/1 nals. The second amplifier provides both integral and proportionalcontrol and drives a firing circuit. By the use of a singleslopeadjustment control, the current [56] References cued and voltagefeedbacksignals may be adjusted to pro- UNITED STATES PATENTS vide infinite andcontinuous control of the voltampere 3,530,359 9/1970 Grist 219/135 Xcharacteristic of the power supply between a constant 5 99 70 Steams etl- 219/135 X current characteristic and a constant potential charac-3-68818O 3/1972 Chlasson a] 219/135 x teristic. An adjustable startcircuit provides control of the average output voltage, and hence theinitial are energy, at the start of operation to more easily establishthe arc.

22v Claims, 4 Drawing Figures I TI 0 POWER tacit. 3 t INTEGRAL PLUS 1/ fsuMmNe PROPORTIONAL TRANSFWMER 12 FILTER AMP. CONTROL AME REFERENCE 4:,-55 SETPOINT 42 52 v wa new.

5 I CK snunr 44 47 /T/' l L, 9" LOAD .a- 17 mecmrgmt .49 v/ mums 2o ARCy.

ELECTRODE 23 1 WORK MEMBER I PATENTED JUL 2 2 ms V VOL'I'S) V VOLTS)Fig. 20

MACHINE CHARACTERISTI IDEAL CONSTANT POWER CHARACTERISTIC VI K IAMPERES) Fig. 20

I l I Q 7 l l 200 300 400 500 600 700 I (AMPERES) DIRECT CURRENT ARCPOWER SUPPLY BACKGROUND OF THE INVENTION l. Field of the Invention 1.

This invention relates to the field of art of direct current are powersupplies having feedback control systems.

1. Prior Art Direct current power supplies are known in which an arc isestablished and maintained between apair of electrodes. The incomingline voltage is reduced in potential by a transformer, the output ofwhich is full wave rectified for establishing a desired direct currentvoltage and current output. Such an arc welding supply is disclosed inUS. Pat. No. 3,549,978 in which a polyphase transformer-rectifier systemincludes silicon controlled rectifiers and has both current and voltagefeedback control. This feedback control is effective to establish thedesired phasing or firing" of the silicon controlled rectifiers. In thispatent, the feedback control system comprises a pair of summingamplifiers, the first of which sums (l) a reference set voltage and (2)the current feedback. The second amplifier sums (1) an output of thefirst amplifier and (2) the voltage feedback. A separate slopeadjustment circuit is provided in the current feedback to establishsubstantially zero slope even though the resistance in the output linetends to establish a drooping characteristic. Such prior power supplieshave left much to be desired in that the range of slope control in theconstant potential mode is limited for stable operation. As a result,such prior supplies have been limited primarily to theshort are or spraytransfer mode of welding where either a constant potential or a slightlydrooping volt-ampere characteristic is desired. v SUMMARY OF THEINVENTION,

In order to overcome the limitations of prior art devices, there isdisclosed herein a direct current are power supply for controlling thevolt-ampere characteristic of direct current applied to establish andmaintain an are between a pair of electrodes. A set reference signal isproduced related to a desired value of direct current. The are currentis sensed to provide an adjustable current feedback signal. In addition,the voltage across the electrodes is sensed to provide an adjustablevoltage feedback signal. A feedback control system sums the setreference signal, the adjustable current feedback signal and theadjustable voltage feedback signal and applies a resultant controlsignal to a power circuit to control the volt-ampere slopecharacteristic. There is simultaneously adjusted both the adjustablecurrent sensing means and the adjustable voltage sensing means toprovide infiniteand continuous control of the volt-ampere characteristicof the power supply between a constant current characteristic and aconstant potential characteristic.

Further in accordance with the invention, the simultaneous adjustment isprovided by a single slope adjustment control in the form of amechanicalgang between a current slope potentiometer and a voltage slopepo-' tentiometer. These two potentiometers are interconnected so thatwith the single slope adjustment control set to one extreme, the currentfeedback signal is maximum and the voltage feedback signal is zerothereby providing a true constant current volt-ampere characteristic.With the single slope adjustment control set to the other extreme, thecurrent feedback signal is zero and the voltagefeedback signalismaximum, thereby providing a true constant potential volt-amperecharacteristic. This single slope adjustment control may be adjustedcontinuously between the .two extremes to yield an infinitely adjustablevolt-ampere slope characteristic. r I

In this manner, the are power supply may be used in applications withoutthe adverse stability characteristics inherent in the prior artsupplies. Thus, the invention may be used not only in the short are orspray transfer modes of welding (Gas Metal Arc Welding) but may be usedequally well for automatic or manual Gas Tungsten Arc Welding andShielded Metal Arc Weldingfl'he are power supply may use solid stateconstruction with high gain feedback operational amplifiers to provide ahighly reliable and long life system in addition to compensating forambient temperature conditions in the circuitry of the feedback controlsystem. As semiconductor devices increase in temperature, they tend todecrease in resistance in direct opposition to the increase inresistance associated with increase in temperature of resistors in thecircuit. Thus in accordance with the invention, there is provided highlydesirable continuously adjustable slope and setpoint control with astable output which is essentially independent of load, line andtemperature conditions.

DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTReferring now to FIG. 1, there is shown an are power supply system 10 inwhich a three phase power source 11 supplies a polyphase weldingtransformer 12. Transformer 12 provides an alternating current input toa full wave polyphase rectifier assembly 14. Rectifier 14 has a pair ofoutput leads 15 and 16 with output lead or load line 16 indicated as thenegative lead and output lead or load line 15 indicated as the positivelead.

In the illustrated embodiment of the supply system of FIG. 1, outputnegative lead 16 is connected to an electrode 20 while output positivelead 15 is connected by way of a shunt 22 to a work member 23. Whensupply system 10 is energized, an are 26 is established and maintainedbetween electrode 20 and work member 23.

The foregoing connections of leads l5 and 16 are 'useful for both themanual and automatic modes of the Gas Tungsten Arc Welding (GTAW or TIG)process for certain modes of the Shielded Metal Arc Welding process(dependent. on the particular type of shielded electrode 20 used).Forthe remaining modes of the Shielded Metal Arc Welding process and forboth the short are and spray transfer'modes of the Gas Metal Arc Weldingprocess,'lead 15 is connected to electrode 20 while lead l6is connectedto work mem ber23.'

For proper arc welding in the manual mode of the Gas Tungsten ArcWelding process, the volt-ampere Characteristic of system 10, as shownin FIG. 2A, is preset so that a desired straight line characteristic 30is substantially tangent to an ideal constant power characteristic (K)31. Voltage V is the voltage across arc 26 taken between leads l5 and 16while current I is the actual current through arc 26. The tangency isprovided at a desired nominal operating point 33. Ideal constant powercharacteristic 31 is a known characteristic in the Gas Tungsten ArcWelding process.

With the volt-ampere characteristic of system set in this manner,automatic compensation for variations in changes of electrode 20 withrespect to work member 23 may be achieved. For example, if electrode 20is moved away from member 23, the voltage across are 26 increases fromits nominal value along characteristic 30 to a point 35, for example.Thus the voltage increases from V, to V This in turn results in adecrease in the arc current I, to I thereby tending to maintain thepower input to are 26 approximately constant.

In the automatic mode of the Gas Tungsten Arc Welding process, a trueconstant current volt-ampere characteristic ,is generally desired. Inthe automatic mode, arc voltage is normally maintained constant by meansof a separate automatic arc voltage controller which controls electrode26, as described for example in US. Pat. No. 2,516,777 For ControlApparatus for Automatic Welding Heads.

In the spray transfer mode of the Gas Metal Arc Welding process, thevolt-ampere characteristic of system 10 is preset normally to either atrue constant potential characteristic or a slightly drooping (negativeslope) characteristic. In the short arc mode of the Gas Metal ArcWelding process, the volt-ampere characteristic of system 10 is presetnormally to a higher magnitude negative slope characteristic than thatin the spray transfer mode. On the other hand, in the Shielded Metal ArcWelding mode, the volt-ampere characteristic is set the same as in themanual mode of the Gas Tungsten Arc Welding process.

Referring again to supply system 10, a reference set point voltage input40 (set in a manner to be described) is applied to one input 42a of asumming filter amplifier 42. A second input 42c of amplifier 42 isconnected to a current feedback unit 45 while a third input 42b toamplifier 42 is taken from a voltage feedback unit 46. In this manner,amplifier 42 is effective to sum the set reference signal, an adjustablecurrent feedback signal and an adjustable voltage feedback signal.

Current feedback unit 45 obtains its current input from a current sensoror shunt 22 which provides a signal proportional to the current throughlead (arc current I). Load resistors 17 are connected between leads l5and 16 and voltage feedback unit 46 is connected across load resistors17 to provide a signal proportional to the voltage across arc 26. Units45 and 46 comprise potentiometers which are mechanically ganged by gang47. The potentiometers are ganged so that with a setting at one extreme(as for example, clockwise) the voltage feedback signal at voltagefeedback conductor 44 is of zero value and the current feedback signalat current feedback conductor 43 is of maximum magnitude. Accordingly,at the output 50 of amplifier 42 there is produced a voltage signalproportional to the setting of reference 40 which at that timeestablishes the desired set point of current.

On the other hand, with gang 47 set at the other extreme (as for examplecounterclockwise), the current feedback signal at conductor 43 is ofzero magnitude while the voltage feedback signal at conductor 44 is at amaximum value.

1 Accordingly, the output of amplifier 42 provides a voltage signalproportional to the setting of reference 40 which establishes thedesired set point of voltage. For intermediate settings of gang 47,output 50 produces modified proportional set point signals related tothe slope characteristic established by the current and voltage feedbacksignals. One of the slope characteristics is shown in FIG. 2A ascharacteristic 30 while another is shown as characteristic 37.

Amplifier 42 also provides low pass filtering characteristic which iseffective to smooth the otherwise high ripple content of the currentfeedback applied to input 42c and the voltage feedback applied toinput42b.

' Output 50 of amplifier 42 is connected to an input of amplifier of 52which provides both integral plus proportional control of the feedbackfor system 10, The output of amplifier 52 is effective to control thefiring networks of firing unit 55. Firing unit 55 is adapted to provideproperly timed and spaced firing pulses to a bank of triggered siliconcontrolled rectifiers in rectifier'14. With this proper phasing ofrectifier 14, there is established and maintained the desired voltageand current characteristics at output leads l5 and 16 and therefore toarc 26.

Inorder to assist in the start of supply system 10, a start circuit 53is effective to provide controlled arc energy at the start of operationto more easily establish arc 26. When the weld current increasessufficiently, a relay is energized thereby removing the start weldsetting so that the weld current assumes its normal desired value asestablished by reference 40.

It will now be understood that current feedback unit 45 comprises aseparate potentiometric slope adjustment connectedbetween shunt 22 andinput 420 while voltage feedback unit 46 comprises a separatepotentiometric slopeadjustment connected between load resistors 17 andinput 42b. The current slope potentiometer is mechanically ganged to thevoltage slope potentiometer.

With the single slope adjustment control (gang 47) setto one extreme,the current feedback signal is maxirr'iurnand the voltage feedbacksignal is zero, thereby providing a true constant current volt-amperecharacteristic. Specifically with gang 47 in its extreme clockwiseposition, the current feedback signal at conductor 43 is at a maximumand the current through arc 26 is maintained substantially constant at avalue determined by the setting of reference. For example as shown inFIG. 28 depending upon the setting of refer ence 40, the current Ithrough are 26 may be amps at one setting, 500 amps at another setting,etc. Each constant current characteristic varies between a voltage valueapproaching zero volts (as for example, 5 volts) to the maximum voltageof supply system 10.

When the single slope adjustment control (gang 47) is set to the otherextreme, the current feedback signal zero and the voltage feedbacksignal is maximum, thereby providing a true constant potentialvolt-ampere characteristic. Specifically, with gang 47 and its extremecounterclockwise position, the voltage feedback at conductor 44 ismaximum and the voltage across are 26 is maintained substantiallyconstant at a voltage value determined by the setting of reference 40.For example as shown in FIG. 2b, the voltage across are 26 may be 20volts at one setting, 40 volts at another setting, etc. with currentvarying from a minimum to a maximum value.

The single slope adjustment control (gang 47) may I be adjustedcontinuously between the two extremes to yield an infinitely adjustablevolt-ampere slope characteristic.

Referring now to FIG. 3, there is shown a detailed drawing of supplysystem in which transformer 12 comprises a three phase delta primary 60and a six phase star secondary 62. The windings of primary 60andsecondary 62 are suitably coupled on a common core (not shown) toprovide a constant potential transformer. Power source 1 1 is coupled toprimary 60. The common 64 or star point of secondary 62 is connected byway of lead or load line through current sensing shunt 22 to work member23. The outer end of each phase winding of secondary 62 is connectedthrough respective silicon controlled rectifiers (SCR) 70a-b, 7l a'-b,72a-b and then through a smoothing and stabilizing reactor 67 and leador load line 16 to electrode 20. Reactor 67 is provided with taps sothat different values may be selected for different rriodes of welding.

It will be understood that diametrically opposite phase windings of thestar connected secondary 62 are connected to simultaneously conductingSCRs 70a-b, 7la-b, and 72a-b. Thus phase 62a is connected to a pair ofSCRs 7 0a-b and then through reactor 67 to line 16. The adjacent phase62b is connected to a similar 'pairof SCRs 7lab and the final phase 620is connected to a final pair of SCRs 72a-b. Each pair of SCRs 70a-b,71a-b, and 72a-b is connected to a firing unit 55 for simultaneouspulsing with the three pairs being pulsed in proper sequence. In thismanner, the portion of the half-wave of each winding applied acrossleads 15-16 is controlled by the phased firing of the respective SCRs.

In the Gas Tungsten Arc Welding process, it is desirable'to utilize abackground supply comprising auxiliary secondary windings 66a-c in themain power transformer 60. Windings 66a-c are .connected in threephaseidelta with the output thereof .coupled to a background rectifier69. Rectifier 69 may be connected as a three phase bridge rectifier. Theoutput of the rectifier 69 is coupled through a switch 69a to leads 15and 16. This circuit operates to help establish a more stable arc bysmoothing the ripple produced by the arc.

Firing unit 55 may be any one of the firing units well known in the arttoprovide the required phase control and firing of SCRs. For exampleunit 55 may be a firing unit Part No. R6l3F372 manufactured by FiringCircuit, Inc., Norwalk, Conn.

Firing unit 55 is actuated by an input conductor 74 which is coupled byway of a gain control potentiometer 75 to output 77 of amplifier 52. Thesignal at conductor 74 determines the particular time in each half cycleat which a firing pulse is applied to a particular SCR and therebydetermine the particular time in the phase that SCR conducts the outputapplied to it from secondary 62. Since power source 11 isalso applied tofiring unit 55, this power input is synchronizedwith the control voltageapplied to an SCR. The firing of the SCR is modified by the input signalat conductor 74 which reflects the current feedback produced by unit 45through summation amplifier 42 thereby to establish a desired voltageand current slope characteristic.

Shunt 22 comprises a millivolt shunt connected in series with lead 15.The upper end 22b of shunt 22 is coupled by way of common lead 80 to thejunction of fixed contacts of current and voltage feedbackpotentiometers 82, 83 respectively. Common is the common for the entireelectronic feedback circuit. The lower end 22a of shunt 22 is coupled byway of conductor 84 to the other fixed contact of current potentiometer82. In this manner, potentiometer 82 is coupled across shunt 22 and thecurrent feedback signal is taken from moveable arm 82a of potentiometer82 with respect to common 80. That current feedback signal is applied toinput 420 of summing filter amplifier 42.

Within amplifier 42, input 420 is coupled to an input circuit 85comprising resistors 80a-c in series and capacitors 97ab. One end of theseries resistance circuit is connected to input 420 and the other end isconnected to a summing junction 92 of an operational amplifier 100 whichcomprises the amplifying device of summing filter amplifier 42. Thefeedback network 102 of operational amplifier 103-106 and capacitors108-111. The values of these resistance and capacitance feedbackcomponents are selected in conjunction with the values of resistors a-cand capacitors 97a-b to provide adequate filtering of the currentfeedback millivolt signal. These components are further selected toprovide proper voltage gain to make the current feedback millivoltsignal compatible with the reference set point voltage applied to input42a and the voltage feedback signal applied to input 42b. The voltagelevels applied to inputs 42a -b may for example each be adjusted toapproximately a 10 volt maximum. level. i

For sensing voltage feedback, a series circuit of a potentiometer 17band a fixed resistor 17a (load resistors 17) is connected between theother fixed contact of potentiometer 83 and load line 16. In thismanner, the potential between load lines 15 and 16 is developed acrossthe series circuit combination of resistors l7a-b and 83 with thevoltage feedback signal taken from arm 83a of potentiometer 83 withrespect to common. By suitably moving arm 83a. the voltage feedbacksignal is adjusted to a level compatible with the set point referencevoltage40: The voltage feedback signal is applied by way of conductor 44to input 42b. Within amplifier 42, input 42b is connected to inputcircuit 86 comprising resistors 9311 -10 in series circuit coupled tojunction 92 and capacitors 98ab. The values of the components of thisresistance-capacitance network are selected in conjunction with thefeedback resistance-capacitance network 102 to provide adequate, unitygain filtering of the voltage feedback signal.

For the reference set point voltage, reference circuit 40 comprises apotentiometer 95 having its arm connected to input 42a. One fixedcontact of the potentiometer is connected to common while the otherfixed contact is connected through a potentiometer 96 to a positivesupply. Input 42a is coupled within amplifier 42 to an input circuit 87comprising resistors 94a-c in series circuit and capacitors 99a-b. Thevalues of these components are selected in conjunction with the valuesof the components of the resistance-capacitance feedback network 102 toprovide a transfer function for the reference input which is compatiblewith the current feedback to input 420 and the voltage feedback to input42b.

100 comprises resistors The ends of input circuits 85-87 remote frominputs 42c, 42b and 42a, respectively are summed at junction 92 which iscoupled to the negative input of operational amplifier 100. Inputcircuits 85-87 and feedback network 102 effectively define a Butterworthfilter network.

An additional input to junction 92 may be traced by way of a resistor116 and then to an arm of a potentiometer 115. One fixed contact ofpotentiometer 115 is connected by way of a resistor 114 to a positivesupply and the other fixed contact is connected to common. Potentiometer115 and resistor 114 form an adjustable voltage divider network used tocalibrate the low end of the reference potentiometer readout in ampereswhen the slope is adjusted to the constant current mode. Potentiometer96, previously described, is used to calibrate the high end of referencepotentiometer 95 readout in amperes when the slope is adjusted to theconstant current mode.

It will be understood that the arms 82a and 83a are ganged together bygang 47 to provide a single slope adjustment control as previouslydescribed. With gang 47 at" its extreme right position (corresponding tothe extreme clockwise position) it will be seen that arm 82a is at itsfurthest position from common while arm 83a is at its closest position.Accordingly, the current feedback is at a maximum and the voltagefeedback is at a minimum. On the other hand, with gang 47 in its extremelefthand position (corresponding to the extreme counterclockwiseposition) arm 82a is at its closest position to common while arm 83a isat its furthest position. Accordingly, the current feedback is zero andthe voltage feedback is maximum.

The output of operational amplifier 100 is applied by way of apotentiometer 120 and a resistor 121 to the negative input of anoperational amplifier 125; the positive input of which is connected byway of a resistor 123 to common. The output of amplifier 125 isconnected by way of an integrator capacitor 130 and a resistor 131 tothe negative input. In this manner, operational amplifier 125 operatesas an augmented integrator, or integral plus proportional amplifier. Inthe steady state if a perturbation occurs in the error signal at theoutput of operational amplifier 100, then capacitor 130 charges in sucha direction as to drive the output of amplifier 125 and hence the firingcircuit 55 to that level necessary to reduce the error signal to zero.Potentiometer 120 may be adjusted to set the closed loop gain of thefeedback circuit.

Start circuit 53 is provided to assist in the start of sup- 1 ply system10. It will be understood that in both manual and automatic welding thatit is desired to have higher weld currents at the start of operation inorder to more easily establish are 26. However, once the arc has beenestablished, it is necessary that the weld current be decreased to itsnormal desired value as established by reference 40.

In order to provide this start level, current feedback by way of theconductors 84 and 129 is applied through a resistor 132 to anegativeinput 135a of an operational amplifier 135. The'current feedbackproduces an effective negative potential at input 135a. Input 135a isalso connected by way of a voltage divider network 136 to a positivesupply. The positive input of amplifier 135 is connected by way of aresistor 136 to common. When the value of the negative potentialproduced by the current feedback reaches a level greater than thepositive potential developed by network 136, then the output ofamplifierjl35 chang es polarity thereby to turn on a switchingtransistor 140 as'shown. The collector of transistor l40,i s coupled toa relay 142, the normally closed contacts 142d'of whi'chare coupledbetween an arm of a potentiometer'l44 and a'diode 145. As shown, thiscircuit is connected between the input and output of amplifier 125,thereby providing'an output clamp on this amplifier. I

Accordingly, at start up relay 142 is deenergiz ed and the normallyclosed contact 142a is effective to apply the start weld level settingof potentiometer 144 to the input of amplifier 125. Thislevel setting isapplied to firing unit 55 for the higher value start up current. Whenthe weld current I increases sufficiently to produce a potential atinput a of amplifier 135 greater than the setting of voltage divider 136than transistor switches thereby energizing relay 142 and opening thecontact 142a. With contact 142a open, the start weld setting is removedand amplifier-52 operates normally.

Having thus described my invention what is desired to be secured byLetters Patent of the United States is set forth in the appended claims.

I claim: 1. An are power supply for controlling the voltamperecharacteristic of the direct current applied by way of lead means to apair of electrodes which establishes and maintains an arc between saidelectrodes, comprising: I

power means operable for providing through said lead means directcurrent to said electrodes,

referencesetpoint means to produce a set reference signal related toadesired value of said direct current, adjustable means connected to saidlead means for sensing said are current and providing an adjustablecurrent feedback signal,

adjustable means connected to said lead means for sensing the voltageacross said electrodes and providing an adjustable voltage feedbacksignal,

feedback control means coupled to said power means for summing said setreference signal, said adjustable current feedback signal and saidadjustable voltage feedback signal for applying a control signal to saidpower means to control said voltampere characteristic, and

means for simultaneously adjusting both said adjustable current sensingmeans and said adjustable voltage sensing means to provide infinite andcontinuous control of said volt-ampere characteristic of said directcurrent between and including a constant current characteristic and aconstant potential characteristic.

2. The are power supply of claim 1 inwhich said simultaneously adjustingrne anscompris es means ganging said adjustable current sensing, meansand said adjustable voltage sensing means to provide a single slopeadjustment control which when set 1) toone extreme said current feedbacksignal is maximum and said voltage feedback signal .is zero therebyproviding said constant current volt-ampere characteristic and (2) toanother extreme said-currentfeedback signal is zero and said voltagefeedback. signal is maximum thereby pro- 3. The arc power supply ofclaim 1 in which said adjustable current sensing means comprises acurrent sensor coupled to said lead meansand voltage potentiometricslope adjustment means coupled to said current sensor for providing saidadjustable current feedback signal proportional to the current flowthrough said lead means, and in which said adjustable voltage sensingmeans comprises load resistor means coupled to said lead means andvoltage potentiometric slope adjustment means coupled to said loadresistor means for providing said adjustable voltage feedback signalproportional to said voltage across said electrodes.

1 4. The arc power supply of claim 3 in which'said simultaneouslyadjusting means comprises rrieans for ganging said current and voltagepotentiometric means so that 1) at one extreme setting of said gangingmeans the current feedback signal is maximum and the voltage feedbacksignal is of zero value to produce a control signal proportional tosaidset reference-signal for establishing a desired setpoint of current, and(2) at another extreme setting of said ganging'means and voltagefeedback signal is maximum and the current feedback signal is of zerovalue to provide-a control signal proportional to the set referencesignal for establishing a desired setpoint of voltage; with intermediatesettings of said ganging means producing proportional control signalsrelated to the slope characteristics established by the current andvoltage feedback signals.

5. The arc power supply of claim 1 in which said feedback control meansincludes first amplifier means for summing said set reference, saidadjustable current and adjustable voltage feedback signals and forproviding a low pass filtering characteristic for Smoothing the highripple content of said feedback signals.

6. The arc power supply of claim 5 in which said feedback control meansincludes second amplifier means connected to the output of said firstamplifier means for providing integral plus proportional control of thefirst amplifier output and for applying the resultant signal as saidcontrol signal to said power means.

7. The are power supply of claim 6 in which said power means comprisesrectifying means having silicon controlled rectifiers for rectifying asource of alternating current for providing said direct current andfiring means adapted to provide properly timed and spaced firing pulsesto rectifiers of said rectifier means under the control of said controlsignal.

8. The arc power supply of claim 5 in which said first amplifier meanscomprises first, second, and third input circuits connected respectivelyto said reference setpoint means, said adjustable current sensing meansand said adjustable voltage sensing means, a first operational amplifierhaving an input connected to a common junction coupling said first,second, and third filter input circuits, a resistance-capacitancenetwork providing feedback for said first operational amplifier, andsaid first, second, and third input citcuits and said feedback networkforming a Butterworth network.

9. The are power supply of claim 8 in which said secwhich in conjunctionwithsaid resistance-capacitance ond input circuit is aresistance-capacitance network a third input circuit is aresistance-capacitance network,

feedback circuit provides adequate unity gain filtering of said voltagefeedback signal.

l l,'The are power supply fof'clair n 10 in which said first inputcircuit is a resistance-capacitance circuit having values selected inconjunction with the values of said resistance-capacitance feedbackcircuit provide a transfer function of the said set reference signalwhich is compatible with the current feedback signal and said voltagefeedback signal.

12. The are power supply of claim 6 in which said second amplifier meansincludes a second operational amplifier having a feedback integratorcapacitor to integrate the applied signal to affect zero error in thesteady state.

13. The are power supply of claim 1 in which there is provided startmeans coupled to said feedback control means for varying said controlsignal by a start level setting at the start of operation to providesaid direct current of value higher than said desired value establishedby said set reference signal.

14. The are power supply of claim 13 in whichsaid start means comprisesstart reference means for establishing a start reference signal, meansconnected to said adjustable current sensing means for comparing anonadjustable current feedback signal with said start reference signalfor (l) applying at the start of operation said start level setting and(2) returning said control signal to its normal value when the arccurrent increases sufficiently to increase the nonadjustable currentfeedback signal to a predetermined value with respect to said startreference signal so that said direct current decreases to its desiredvalue established by said set reference signal.

15. The arc power supply of claim 14 in which said comparing meanscomprises a third operational ampli-' fier and a relay connected to anoutput of said third operational amplifier, said relay having normallyclosed contacts connected in circuit with said feedback control meansfor applying said start level setting when said contacts are closed andfor removing said start level setting when said contacts are opened uponactuation of said relay by said third operational amplifier.

16. A direct current are power supply for establishing and maintainingan are between a pair of electrodes in which upon start of operation ahigher than normal valued startup direct current is applied to saidelectrodes, comprising power means operable for providing direct currentto said electrodes,

reference setpoint means to produce a set reference signal related to adesired value of said direct current,

means for sensing said arc current and providing a current feedbacksignal,

means for sensing the voltage across said electrodes and providing avoltage feedback signal,

feedback control means coupled to said power means for summing said setreference signal, said current feedback signal and said voltage feedbacksignal for applying a control signal to said power means to control thevolt-ampere characteristic of said direct current,

start reference means to produce a start reference signal, and

start means for comparing said current feedback signal with said startreference signal for varying said control signal upon startup to producesaiddirect current of value higher than said desired value which valuedecreases to said desired value when said current feedback signalincreases to a predetermined value with respect to said start referencesignal.

17. The are power supply of claim 16 in which said start means includesan operational amplifier having one input coupled to said currentsensing means. relay means connected to an output of said operationalamplifier and having a normally closed relay contact. start levelsetting means connected to said normally closed relay contact and tosaid feedback control means for varying said control signal by a startlevel setting until said current feedback signal increases to saidpredetermined value at which time said relay is actuated.

18. The arc power supply of claim 17 in which said feedback controlmeans includes an operational amplifier having an integrator capacitorconnected in a feedback loop therewith and providing at an output saidcontrol signal, said start level setting means coupled in anotherfeedback loop whereby upon startup a start level setting is applied tothe input of said operational amplifier until said current feedbacksignal increases to said predetermined value.

19. A method of controlling the volt-ampere charac- 12 ing an adjustablevoltage feedback signal related to the value of the electrode voltage,summing the set reference signal, the adjustable current feedback signaland the adjustable voltage feedback signal and controlling thevolt-ampere characteristic as a function of these signals, andsimultaneously varying both the adjustable current feedback signal andthe adjustable voltage feedback signal to provide an infinite andcontinuous control of the volt-ampere slope characteristic of the directcurrent between and including a con-. stant current mode and a constantpotential mode.

20. The method of claim 19 in which the simultaneously varying stepincludes simultaneously varying the signals so that (l) at one extremethe adjustable current feedback signal is maximum and the adjustablevoltage feedback signal is zero thereby providing a constant currentvolt-ampere characteristic and (2) at another extreme the adjustablecurrent feedback signal is zero and the adjustable voltage feedbacksignal is maximum thereby providing a constant potential volta'mperecharacteristic.

' 21. The method of claim 19 in which the summing step includesproviding a low pass filtering characteristic for smoothing the highripple content of said adjustable current and voltage feedback signalsand then providing integral plus proportional control.

2-2. The method of claim 19 in which the summing step includes varyingthe controlling of the volt-ampere characteristic by a start levelsetting at the start of oper-v ation to provide the direct current ofvalue higher than said desired value established by said set referencesignal.

1. An arc power supply for controlling the volt-ampere characteristic ofthe direct current applied by way of lead means to a pair of electrodeswhich establishes and maintains an arc between said electrodes,comprising: power means operable for providing through said lead meansdirect current to said electrodes, reference setpoint means to produce aset reference signal related to a desired value Of said direct current,adjustable means connected to said lead means for sensing said arccurrent and providing an adjustable current feedback signal, adjustablemeans connected to said lead means for sensing the voltage across saidelectrodes and providing an adjustable voltage feedback signal, feedbackcontrol means coupled to said power means for summing said set referencesignal, said adjustable current feedback signal and said adjustablevoltage feedback signal for applying a control signal to said powermeans to control said voltampere characteristic, and means forsimultaneously adjusting both said adjustable current sensing means andsaid adjustable voltage sensing means to provide infinite and continuouscontrol of said volt-ampere characteristic of said direct currentbetween and including a constant current characteristic and a constantpotential characteristic.
 2. The arc power supply of claim 1 in whichsaid simultaneously adjusting means comprises means ganging saidadjustable current sensing means and said adjustable voltage sensingmeans to provide a single slope adjustment control which when set (1) toone extreme said current feedback signal is maximum and said voltagefeedback signal is zero thereby providing said constant currentvolt-ampere characteristic and (2) to another extreme said currentfeedback signal is zero and said voltage feedback signal is maximumthereby providing said constant potential volt-ampere characteristic. 3.The arc power supply of claim 1 in which said adjustable current sensingmeans comprises a current sensor coupled to said lead means and voltagepotentiometric slope adjustment means coupled to said current sensor forproviding said adjustable current feedback signal proportional to thecurrent flow through said lead means, and in which said adjustablevoltage sensing means comprises load resistor means coupled to said leadmeans and voltage potentiometric slope adjustment means coupled to saidload resistor means for providing said adjustable voltage feedbacksignal proportional to said voltage across said electrodes.
 4. The arcpower supply of claim 3 in which said simultaneously adjusting meanscomprises means for ganging said current and voltage potentiometricmeans so that (1) at one extreme setting of said ganging means thecurrent feedback signal is maximum and the voltage feedback signal is ofzero value to produce a control signal proportional to said setreference signal for establishing a desired setpoint of current, and (2)at another extreme setting of said ganging means and voltage feedbacksignal is maximum and the current feedback signal is of zero value toprovide a control signal proportional to the set reference signal forestablishing a desired setpoint of voltage; with intermediate settingsof said ganging means producing proportional control signals related tothe slope characteristics established by the current and voltagefeedback signals.
 5. The arc power supply of claim 1 in which saidfeedback control means includes first amplifier means for summing saidset reference, said adjustable current and adjustable voltage feedbacksignals and for providing a low pass filtering characteristic forsmoothing the high ripple content of said feedback signals.
 6. The arcpower supply of claim 5 in which said feedback control means includessecond amplifier means connected to the output of said first amplifiermeans for providing integral plus proportional control of the firstamplifier output and for applying the resultant signal as said controlsignal to said power means.
 7. The arc power supply of claim 6 in whichsaid power means comprises rectifying means having silicon controlledrectifiers for rectifying a source of alternating current for providingsaid direct current and firing means adapted to provide properly timedand spaced firing pulses to rectifiers of said rectifier means under thecontrol of said control signal.
 8. The arc power supply oF claim 5 inwhich said first amplifier means comprises first, second, and thirdinput circuits connected respectively to said reference setpoint means,said adjustable current sensing means and said adjustable voltagesensing means, a first operational amplifier having an input connectedto a common junction coupling said first, second, and third filter inputcircuits, a resistance-capacitance network providing feedback for saidfirst operational amplifier, and said first, second, and third inputcitcuits and said feedback network forming a Butterworth network.
 9. Thearc power supply of claim 8 in which said second input circuit is aresistance-capacitance network which together with saidresistance-capacitance feedback circuit provide (1) adequate filteringof said current feedback signal and (2) proper voltage gain so that saidcurrent feedback signal is compatible with said set reference signal andsaid voltage feedback signal.
 10. The arc power supply of claim 9 inwhich said third input circuit is a resistance-capacitance network whichin conjunction with said resistance-capacitance feedback circuitprovides adequate unity gain filtering of said voltage feedback signal.11. The arc power supply of claim 10 in which said first input circuitis a resistance-capacitance circuit having values selected inconjunction with the values of said resistance-capacitance feedbackcircuit provide a transfer function of the said set reference signalwhich is compatible with the current feedback signal and said voltagefeedback signal.
 12. The arc power supply of claim 6 in which saidsecond amplifier means includes a second operational amplifier having afeedback integrator capacitor to integrate the applied signal to affectzero error in the steady state.
 13. The arc power supply of claim 1 inwhich there is provided start means coupled to said feedback controlmeans for varying said control signal by a start level setting at thestart of operation to provide said direct current of value higher thansaid desired value established by said set reference signal.
 14. The arcpower supply of claim 13 in which said start means comprises startreference means for establishing a start reference signal, meansconnected to said adjustable current sensing means for comparing anonadjustable current feedback signal with said start reference signalfor (1) applying at the start of operation said start level setting and(2) returning said control signal to its normal value when the arccurrent increases sufficiently to increase the nonadjustable currentfeedback signal to a predetermined value with respect to said startreference signal so that said direct current decreases to its desiredvalue established by said set reference signal.
 15. The arc power supplyof claim 14 in which said comparing means comprises a third operationalamplifier and a relay connected to an output of said third operationalamplifier, said relay having normally closed contacts connected incircuit with said feedback control means for applying said start levelsetting when said contacts are closed and for removing said start levelsetting when said contacts are opened upon actuation of said relay bysaid third operational amplifier.
 16. A direct current arc power supplyfor establishing and maintaining an arc between a pair of electrodes inwhich upon start of operation a higher than normal valued startup directcurrent is applied to said electrodes, comprising power means operablefor providing direct current to said electrodes, reference setpointmeans to produce a set reference signal related to a desired value ofsaid direct current, means for sensing said arc current and providing acurrent feedback signal, means for sensing the voltage across saidelectrodes and providing a voltage feedback signal, feedback controlmeans coupled to said power means for summing said set reference signal,said current feedback signal and said voltage feedback signal forapplying a control signal to said power means to control the volt-amperecharacteristic of said direct current, start reference means to producea start reference signal, and start means for comparing said currentfeedback signal with said start reference signal for varying saidcontrol signal upon startup to produce said direct current of valuehigher than said desired value which value decreases to said desiredvalue when said current feedback signal increases to a predeterminedvalue with respect to said start reference signal.
 17. The arc powersupply of claim 16 in which said start means includes an operationalamplifier having one input coupled to said current sensing means, relaymeans connected to an output of said operational amplifier and having anormally closed relay contact, start level setting means connected tosaid normally closed relay contact and to said feedback control meansfor varying said control signal by a start level setting until saidcurrent feedback signal increases to said predetermined value at whichtime said relay is actuated.
 18. The arc power supply of claim 17 inwhich said feedback control means includes an operational amplifierhaving an integrator capacitor connected in a feedback loop therewithand providing at an output said control signal, said start level settingmeans coupled in another feedback loop whereby upon startup a startlevel setting is applied to the input of said operational amplifieruntil said current feedback signal increases to said predeterminedvalue.
 19. A method of controlling the volt-ampere characteristic ofdirect current applied to establish and maintain an arc between a pairof electrodes which comprises producing a set reference signal relatedto a desired value of said direct current, sensing the arc current andproviding an adjustable current feedback signal related to the value ofthe arc current, sensing the voltage across the electrodes and providingan adjustable voltage feedback signal related to the value of theelectrode voltage, summing the set reference signal, the adjustablecurrent feedback signal and the adjustable voltage feedback signal andcontrolling the volt-ampere characteristic as a function of thesesignals, and simultaneously varying both the adjustable current feedbacksignal and the adjustable voltage feedback signal to provide an infiniteand continuous control of the volt-ampere slope characteristic of thedirect current between and including a constant current mode and aconstant potential mode.
 20. The method of claim 19 in which thesimultaneously varying step includes simultaneously varying the signalsso that (1) at one extreme the adjustable current feedback signal ismaximum and the adjustable voltage feedback signal is zero therebyproviding a constant current volt-ampere characteristic and (2) atanother extreme the adjustable current feedback signal is zero and theadjustable voltage feedback signal is maximum thereby providing aconstant potential volt-ampere characteristic.
 21. The method of claim19 in which the summing step includes providing a low pass filteringcharacteristic for smoothing the high ripple content of said adjustablecurrent and voltage feedback signals and then providing integral plusproportional control.
 22. The method of claim 19 in which the summingstep includes varying the controlling of the volt-ampere characteristicby a start level setting at the start of operation to provide the directcurrent of value higher than said desired value established by said setreference signal.